Currently, ships and task group movements cna be detected by a hostile interceptor using relatively simple receivers in aircraft and satellites. Detection can be effective beyond many times the range of the radiating equipment. A Navy concept known as the Quiet Task Force (QTF), provides the option to shut down Navy ship sources of radiation and use land based over-the-horizon radar and other means of surveillance. This concept provides that ships go active immediately upon knowledge of imminent threat or at times when ship locations and activities are not required to be concealed.
Three gaps in this concept are (1) surface search for navigation, collision avoidance and station keeping, (2) point defense against short range low flyer attack, and (3) air traffic control capability.
Basic considerations of radar intercept resistance indicate that conventional low probability of intercept (LPI) techniques will not provide adequate intercept resistance to a radar which meets the above objectives. Conventional techniques such as high processing gain and spread spectrum, though useful for LPI communication, cannot overcome the 1/R.sup.4 radar signal decrease versus that the 1/R.sup.2 one-way intercept signal decrease. In addition, radar requirements limit the amount of signal processing gain which can be accommodated.
A signal intercept is made when the signal available, S.sub.a, equals or exceeds the intercept receiver sensitivity, S.sub.i. The problem now faced by Navy ships is that for actual radar transmitter power and antenna gains required for target detection, the available signal to the intercept receiver is sufficiently high that interception is limited only by the radar horizon. Interception below the horizon is possible in many situations. It has also been shown analytically that the use of very large radar processing gain cannot tip the balance in favor of the radar except for large, slow moving radar targets such as ship targets. The principal reason is the 1/R.sup.4 decrease in radar receiver signal power compared to the 1/R.sup.2 decrease in intercept receiver signal power. High signal processing gain while providing some sub-clutter visibility probably cannot be made to adequately handle high sea states. High speed targets also limit the amount of processing gain that can be used.